The invention pertains to a image sensor with memory and more especially to an image sensor with memory with reading by thermal effect.
The field of the invention is that of image sensors with memory combining a photoconductor and a dielectric wherein the addressing during reading is done by laser-beam scanning, the said laser beam setting up, in a circuit containing the sensor, a photocurrent which is proportionate to the illumination received at the addressed point. Two publications are relevant in this field. They are:
(1) "A Ferroelectric Image Memory" by F. MICHERON, J. M. ROUCHON and M. VERGNOLLE (THOMSON CSF) published in Ferroelectrics 10, 15 (1976)
(2) U.S. Pat. No. 4,446,365 filed on May 1, 1984, with the title of "Electrostatic Imaging Method" by P. S. ONG, A. ZERMENO, L. M. MARSH and J. M. HEVEZI
The first of these publications describes an image sensor wherein the photoconductor is associated with a ferroelectric dielectric. When recording, since the ferroelectric is initially in the macroscopically depolarized state, well above its Curie point temperature (random direction of the polar axes of the microcrystals which constitute it), it is subjected simultaneously to the application, via the photoconductor, of a voltage greater than the coercitive voltage and to the projection of the image to be recorded. In the highly illuminated zones the voltage is applied, in totality, to the ferroelectric, and the crystals take a direction. In the grey zones, the rate at which the direction is taken depends on the illumination received. This results in a replica of the image in the ferroelectric, recorded in the form of a spatial distribution of polarization, and this image is permanent for as long as the ferroelectric is not heated to its Curie point temperature or for as long as it is not subjected to the coercitive voltage. The reading is done by scanning with a laser beam, either on the photoconductive side (in this case, there flows, in the external circuit, a photocurrent caused by the displacement of the photocarriers which neutralizes the space charge created by the ferroelectric at the interface) or on the ferroelectric side, in which case a current of pyroelectric or photoferroelectric origin flows in the circuit. These currents constitute the video signal sent to a cathode-ray tube display device, scanned in synchronism with the laser scanning, hence the restitution of the image. It will be noted that, in this type of image memory, the reading is nondestructive.
The latter of the two publications cited above pertains to the same principle, except that the dielectric does not have the property of memory and that the reading brings into play only the mode of neutralization of the space charge by the illuminated photoconductor. In this case, the storage time of the recorded image depends on the natural discharge, time constants (Maxwell's relaxation) of the photoconductor and the dielectric. Finally the reading is destructive.
The image sensor proposed in the present patent application is intermediary between the two preceding sensors since the reading is nondestructive and since the storage time is monitored by the Maxwell relaxation time of the photoconductor and the dielectric. The reading is done through the impedance variation induced thermally in the dielectric. The physical bases of this mode of reading are explained in the following publication:
(3) "Proprietes et Applications des Dielectrics Polarises: Electrets, Polymers Orientes et Ferroelectrics" (Properties and Applications of Polarized Dielectrics: Electrets, Directed Polymers and Ferroelectrics), by F. MICHERON, in the Revue Technique THOMSON CSF, No. 10, p. 445 (1978).